This invention relates generally to programmable signal generators and more particularly to precision signal generators for precise triangular waveform generation.
In various precision measurement applications such as a high resolution mass spectrometry application, a precision triangular generator is often needed. It is required that the waveform generator be stable for a period of several hours in order to acquire mass resolution data buried in the system noise.
In a digital approach to designing a triangular waveform generator, an oscillator drives a counter and the counter output goes to a digital-to-analog converter (DAC). The output from the DAC is a staircase ramp rather than a linear ramp. In many applications, as described above, it is desirable that the linearity of the ramp be very precise, thus eliminating the digital step approach of generating a ramp signal or signals for a triangular waveform.
Another approach to designing a triangular waveform generator is by analog means. In the conventional analog design, two constant current sources are used to generate the ascending and descending ramps. Due to the current sources being switched at their outputs, switching transient voltages are produced in the output waveform that would be intolerable in precision measurement applications. A second difficulty with the conventional analog approach is a need to design two constant current sources having such precision that a great deal of complexity and cost is involved. Some analog integrator designs use field effect transistors (FET) as switches. The characteristics of these devices limit the range of useable integrator time constants (ramp rates) where precision performance is required. Thus, there is a need for an instrument to provide a precise triangular waveform without the drawbacks of the digital waveform generator and the complexity and cost of available precision analog waveform generators.